Digital Light Processing 3D‐Printed Silica Aerogel and as a Versatile Host Framework for High‐Performance Functional Nanocomposites

Abstract Vat‐photopolymerization‐based 3D printing enables on‐demand construction of customized objects with scalable production capacity and high precision. Herein, the sol‐gel process for aerogels with digital light processing 3D printing to produce advanced functional materials possessing hierarchical pore structures and complex shapes is combined. It has revealed the temporal evolution of the photorheological behavior of acrylate‐modified silica sols in an acid‐base catalytic procedure, and confirmed that silica aerogels can be fabricated with very low acrylate content. The resulting aerogels are thermostable with intrinsic silica contents, skeletal densities, and physical characteristics similar to those of commercial silica aerogels yet distinct mechanical behaviors. More importantly, the printed silica aerogels can be used as a versatile nanoengineering platform to produce high‐performance and multifunctional interpenetrating phase nanocomposites with complex shapes through programmable post‐printing processes. Epoxy‐based nanocomposites possessing excellent mechanical performance, ionogel‐based conductive nanocomposites with decoupled electrical and mechanical properties, and anti‐swelling hydrogel‐based nanocomposites are demonstrated. The results of this study offer new guidelines for the design and fabrication of novel materials by additive manufacturing.


Experimental Section
Preparation of acrylate-modified silica sols: Ethanol of 92.5 g and 0.1 mol L −1 HCl aqueous solution (pH~1) of 36.2 g were mixed in a round-bottom flask and magnetically stirred for 5 min. TEOS (Acros) was added dropwise to the above solution while stirring, and the reaction was conducted at room temperature for 24 h.
Subsequently, MAPTMS (Sigma-Aldrich) was added, and the reaction continued for another 48 h for the formation of acrylate-modified silica sol. The amounts of TEOS and MAPTMS are summarized in Table S1. For example, the volumes of TEOS and MAPTMS were 84 and 6 mL, respectively, for the sol of group 282.
DLP-printed silica aerogels: A typical formulation of printing ink was as follows. In group 282, 0.2 wt% HMTA (Aladdin; dissolved in 1 g equimolar ethanol/water mixture) and 0.335 wt% diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO, photoinitiator, J&K Scientific) were added, followed by stirring and heating at 50 °C for 2 h. Printing inks were obtained after the sol was cooled to room temperature, and 0.015 wt% dye of phenol red sodium (Innochem) was added. The same procedure was used to prepare the ink of group 273, except 0.5 wt% photoinitiator, and heating time of 3 h were used.
Then, the mixture was diluted to half of its original concentration by an ethanol/water mixture (molar ratio 3.8:1) to obtain group 273-half (with 0.0125 wt% dye added). 3D printing was performed with a commercial DLP printer (Asiga Max X27, equipped with a DMD with a pixel resolution of 1920 × 1080 and a 385 nm UV LED lamp with a settled output light intensity of 27 mW cm −2 ) through a bottom-up layer-by-layer building process in the vertical direction. The printing layer thickness was 100 μm for burn-in layers and 200 μm for normal layers, and the exposure time was approximately 30-40 s. The dye system was changed to a mixture of Rhodamine B (Acros, typical value of 0.03 wt%) and propyl gallate (J&K Scientific, typical value of 0.15 wt%), [1] and the exposure time of the normal layer was adjusted to 13 s with an exposure light intensity of 18 mW cm −2 during the printing process of complex hollow gel objects using group 282. After printing, the objects were post-cured at least 30 min in an equimolar ethanol/water mixture with 0.2 wt% HMTA and under irradiation of a 385 S3 nm UV LED lamp with an output power of ~10 mW cm −2 , followed by aging in an oven at 80 °C for 5 h. The aged objects were subjected to solvent exchange with ethanol 6 times (8-12 h each) at room temperature and hydrophobic treatment in a hexamethyldisilazane (Acros) solution (15 wt% in ethanol) at 60 °C for 24 h. All samples were dried by a supercritical CO2 dryer (ShiAnJia (Beijing) Biotechnology).
Hydrophilic samples were dried directly after solvent exchange without the hydrophobization treatment.
Isocyanate-modified DLP-printed silica aerogels: The modification followed a previously reported method. [2] In brief, after washing with ethanol (4 times, 8-12 h each) and acetone (4 times, 8-12 h each), the printed wet gels were immersed in a poly(hexamethylene diisocyanate) (Sigma-Aldrich)/acetone solution, equilibrated under frequent agitation for 24 h, heated to 55 °C and held for 48 h. Isocyanate/acetone solutions of 1) 5/95 with a total mass of 20 g, 2) 50/50 with a total mass of 20 g, and 3) 50/50 with a total mass of 50 g were used to control the amount of isocyanate reacted on the skeleton. Different volumes of 3D printed wet gel objects were modified in the isocyanate/acetone solution maintaining the same ratios (v/w) of gel volume and solution mass. Afterward, the samples were cooled to room temperature and washed with fresh acetone (4 times, 8-12 h each). All isocyanate-modified samples were dried with a supercritical CO2 dryer.
The resin was replaced three more times for 24 h each. Next, the samples were removed from the container, excess resin on the surface was wiped off, and the samples were transferred to an oven for gradient thermal curing (90 °C for 2 h and 150 °C for 7 h).
The pristine epoxy-anhydride resin was cured under the same curing conditions. For ionogel-silica aerogel nanocomposites, after ethanol washing, the aged DLP-printed   As the content of isocyanate increased, the bulk density of the modified aerogel increased, the specific surface area (SBET) and the pore volume (Vtotal) decreased, and the average pore size (Dp,BJH) increased. These results indicated that after isocyanate modification, the as-formed polymer coated the native skeleton of silica aerogel and Vp,BJH (cm 3 g −1 ) Dp,BJH  Figure S1.  linear shrinkage, which was attributed to the free volume reduction due to polymerization in 3D printing and subsequent aging. S15 Figure S7. Images of water droplets on the surface of DLP-printed objects from group 282 (a) and 273-half (b). S16 Figure S8. TGA data for DLP-printed silica aerogels after hydrophobization. The weight loss at > 300 °C was mainly derived from the organic groups of MAPTMS and ≡Si-CH3. [3,4]  reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) spectra of the modified aerogels. SEM images showed that the pearl-necklace-like structure was preserved, but the skeleton was significantly thicker with increasing isocyanate content, which was consistent with the results of the N2 sorption/desorption test, as shown in Table S2. The ATR-FTIR spectra confirmed the presence of diazetidinedione (~1767 cm −1 , from isocyanate) and carbamate (or urea group/isocyanurate ring with overlapping absorption peak positions, ~1690 cm −1 ) in all modified aerogels. [2,8] The carbamate was formed by the reaction of silanol group with isocyanate. The urea group was formed by the reaction between isocyanates after being hydrolyzed into an amine group by water confined on the skeleton surface. The isocyanurate ring was formed by the condensation of the isocyanate). Besides, Si-O-Si bonds (~1078 cm −1 ) were present in all modified aerogels, and unreacted isocyanates (~2270 cm −1 ) were not observed.